Field of the Invention
The invention relates generally to downhole tools for use in wells, and more particularly to means for determining the proper phase rotation for power that is supplied to a downhole linear motor.
Related Art
In the production of oil from wells, it is often necessary to use an artificial lift system to maintain the flow of oil. The artificial lift system commonly includes an electric submersible pump (ESP) that is positioned downhole in a producing region of the well. The ESP has a motor that receives electrical signals from equipment at the surface of the well. The received signals run the motor, which in turn drives a pump to lift the oil out of the well.
ESP motors commonly use rotary designs in which a rotor is coaxially positioned within a stator and rotates within the stator. The shaft of the rotor is coupled to a pump, and drives a shaft of the pump to turn impellers within the body of the pump. The impellers force the oil through the pump and out of the well. While rotary motors are typically used, it is also possible to use a linear motor. Instead of a rotor, the linear motor has a mover that moves in a linear, reciprocating motion. The mover drives a plunger-type pump to force oil out of the well.
In order to properly control a linear motor, it is desirable to know the electrical position of the mover within the stator. Linear motors may use several sensors (e.g., Hall-effect sensors) to determine the electrical position and absolute position of the mover. The signals from these sensors are provided to a control system, which then produces a drive signal based upon the position of the mover and provides this drive signal to the motor to run the motor.
An ESP using a linear motor typically operates on three-phase power. Each phase is carried by a separate conductor, and is typically shifted by 120 degrees from the other phases. An electrical drive system at the surface of the well generates the three-phase drive signal that is supplied to the motor, which in turn drives the pump. When the system is installed, it is commonly necessary to make various connections (e.g., cable splices) between the electrical conductors that convey the electrical power to the motor. It is not unusual for mistakes to be made in these connections, resulting in electrical connections between the electrical drive system and pump motor that are incorrect. More specifically, two or more of the conductors may be switched. Such misconnection of the conductors may also occur when maintenance is performed on the electrical drive system or the cabling.
Because the phasing of a three-phase electrical signal is reversed (e.g., A-B-C becomes C-B-A) when any two of the three wires are switched, misconnection of these wires can result in the pump motor being driven in a direction which is opposite the intended direction. In other words, when the electrical drive system produces a drive signal with phasing that is intended to drive the motor in the forward direction, it actually drives the motor in the reverse direction. In the case of a linear motor, the drive's output signal is intended to drive the upstroke/downstroke of the motor, so if the phase rotation is reversed, the mover will be driven upward when it is intended to be driven downward, and downward when it is intended to be driven upward. While this may still result in some fluid being produced from the well, it typically is not as efficient as if the proper phasing is used. Additionally, if the motor is intended to be driven in a particular manner on upward or downward strokes (e.g., faster on the downward stroke), this will actually occur on the opposite stroke.
It would therefore be desirable to provide improved means for determining the phasing at the output of the drive that is associated with a linear motor's upstroke and downstroke, and for utilizing this information to generate signals to drive the linear motor.